Bipolar plate/diffuser for a proton exchange membrane fuel cell

ABSTRACT

A combination bipolar plate/diffuser fuel cell component includes an electrically conducting solid material having: 
     a porous region having a porous surface; and 
     a hermetic region, 
     the hermetic region defining at least a portion of at least one coolant channel, the porous region defining at least a portion of at least one reactant channel, the porous region defining a flow field medium for diffusing the reactant to the porous surface.

This is a division of application Ser. No. 08/732,513, filed Oct. 15,1996, now U.S. Pat. No. 6,037,073.

The United States Government has rights in this invention pursuant tocontract no. DE-AC05-84OR21400 between the United States Department ofEnergy and Lockheed Martin Energy Systems, Inc., and contract no.DE-AC05-96OR22464 between the United States Department of Energy andLockheed Martin Energy Research Corporation.

FIELD OF THE INVENTION

The present invention relates to fuel cells for producing electricity,and more particularly to monolithic combination bipolar plate anddiffusers for such fuel cells.

BACKGROUND OF THE INVENTION

Fuel cells, especially proton exchange membrane fuel cells (PEMFC) areknown to generally comprise a number of component layers that provideelectrical contact (electrodes); channels for coolant, fuel, andoxidant; diffusion layers for dispersing the fuel and oxidant; acatalytic element for each pole; and the electrolyte membrane.

In the manufacture of such fuel cells, consideration of the cost offabricating and assembling multiple components and the ohmic lossesacross interfaces would appear to encourage the combination offunctions. Yet the components described above are currently produced asdiscrete elements that require assembly into a unit stack. Moreover, oneof the most costly components is the bipolar plate, which is currentlymachined from graphite.

For further background information, please refer to the followingpublications:

1. Stinton, et al., U.S. Pat. No. 5,075,160, Dec. 24, 1991.

2. Lackey, et al., U.S. Pat. No. 4,580,524, Apr. 8, 1986.

3. K. Kinoshita, F. R. McLarnon, and E. J. Cairns, Fuel Cells: Handbook,DOE/METC-88/6096, pp. 2-4, Lawrence Berkeley Laboratory, Berkeley,Calif., May, 1988.

4. R. Lemons, J. Eberhardt, A. Landgrebe, D. MacArthur, R. Savenell, S.Swathirajan, D. Wilson, and M. Wilson, “Batteries and Fuel Cells,”Current Status, Research Needs, and Opportunities in Applications ofSurface Processing to Transportation and Utilities Technologies:Proceedings of a December 1991 Workshop, A. W. Czanderna and A. R.Landgrebe, Editors, NREL/CP-412-5007, pp. 21-1-21-14, National RenewableEnergy Laboratory, Golden, Colo. September 1992.

5. M. C. Kimble and N. E. Vanderborgh, “Reactant Gas Flow Fields inAdvanced PEM Fuel Cell Designs,” Proceedings of the 27th IntersocietyEnergy Conversion Engineering Conference, Vol. 3, pp. 3.413-3.417,Society of Automotive Engineers, Warrendale, Pa. (1992).

6. K. Strasser, “PEM Fuel Cells for Energy Storage Systems,” pp.630-635, Proceedings of the 26th Intersociety Energy ConversionEngineering Conference, Vol. 3, American Nuclear Society, La GrangePark, Ill. (1991).

OBJECTS OF THE INVENTION

Accordingly, objects of the present invention include the provision of anew and improved fuel cell in which the bipolar plate and diffuser arecombined into a single monolithic component, two series cells (anode andcathode) are optionally combined into a single monolithic component, andsimple geometry, resulting in less costly construction and lower ohmiclosses.

Further and other objects of the present invention will become apparentfrom the description contained herein.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, the foregoingand other objects are achieved by a monolithic combination bipolarplate/diffuser fuel cell component which includes an electricallyconducting solid material having a porous region having a porous surfaceand a hermetic region, the hermetic region defining at least a portionof at least one coolant channel, the porous region defining at least aportion of at least one reactant channel, the porous region defining aflow field medium for diffusing the reactant to the porous surface.

In accordance with another aspect of the present invention, a monolithiccombination bipolar plate/diffuser fuel cell component includes anelectrically conducting solid material having: a first porous regionhaving a first porous surface; a second porous region having a secondporous surface; and a hermetic region, the hermetic region defining atleast one coolant channel, the first porous region defining at least aportion of at least one fuel channel, the second porous region definingat least a portion of At least one oxidant channel, the first porousregion defining a flow field medium for diffusing the fuel to the firstporous surface, the second porous region defining a flow field mediumfor diffusing the oxidant to the second porous surface.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a sectional view of monolithic combination single electrodebipolar plate/diffuser in accordance with the present invention.

FIG. 2 is a sectional view of stacked fuel cells using a series ofmonolithic anode and cathode bipolar plate/diffusers in accordance withthe present invention.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawings.

DETAILED DESCRIPTION OF THE INVENTION

In a more simple embodiment of the present invention, shown in FIG. 1, asingle monolithic electrode/diffuser fuel cell component 10 serves asboth the bipolar plate and the diffuser thereof. The component 10 isinitially fabricated from a conductive, porous base material, preferablya fibrous material, more preferably fibrous carbon. The base material isselected on the basis of porosity which has the appropriate diffusioncharacteristics for the fuel and oxidant. It should be apparent to theskilled artisan that various conventional base materials and fabricationtechniques can be employed to fabricate a conductive preform having aselected porosity.

In the preferred embodiment of the present invention, the basic(component material is molded to an appropriate shape by conventionalslurry molding techniques using chopped or milled carbon fibers ofvarious lengths. Such a method can be carried out as follows:

Step 1. An aqueous slurry is prepared which comprises a mixture ofcarbon fibers having lengths typically in the range of about 0.1 mm toabout 100 mm and about 20 wt % to about 50 wt % phenolic resin powderbinder.

Step 2. The slurry is forced through an appropriate mesh size screen totrap the solids, thus producing a wet monolith which is subsequentlydried at a temperature of less than 80° C. The initial porosity, in theslurry molded and dried condition, is typically in the range 70-90%.

Step 3. The dried monolith is further densified and the resin is curedvia conventional means such as mechanical compression at low pressure inshaped graphite molds at a temperature in the range of about 120° C. toabout 160° C., preferably about 130° C.

Step 4. The densified, cured monolith is pyrolized (carbonized) at atemperature in the range of about 700° C. to about 1300° C. in an inertenvironment. In the pyrolized condition the resultant total porosity istypically in the range of about 40% to about 60%, and the pore sizerange is typically about 10 to about 100 microns. This is a suitablepore structure for subsequent CVI processing to produce a hermeticregion 12 while retaining sufficient open porosity in a region 14remaining porous to allow diffusion of the reactant gasses through thebipolar plate.

A preferably planar region of the material is densified to a non-poroushermetic state and called the hermetic region 12. It is necessary toseal a region of the preform that contains the coolant channels 18 inorder to contain coolant therein and to prevent transport of fuel oroxidant toward the wrong electrode of the fuel cell. Densification tothe hermetic state is preferably achieved via a conventional chemicalvapor infiltration (CVI) technique.

For example, the porous region 14 which is not to be densified is maskedvia a conventional masking technique. The component is then contactedwith a hydrocarbon gas, typically diluted in an inert gas, at reducedpressure and at a temperature in the range of about 800° C. to about1500° C. The hydrocarbon infiltrates the exposed region of thecomponent, reacts and deposits carbon on the fibers, and when sufficientdeposition has occurred the region becomes hermetic. Infiltrated carbonfurther provides additional electrical conductivity.

The remaining region of undensified material remains porous and iscalled the porous region 14. The hermetic region defines coolantchannels 16, and the porous region defines at least portions of reactantchannels 18. The depth of the hermetic region 12 is controlled duringfabrication to avoid surrounding (and subsequently sealing) the reactantchannels 18. The hermetic region 12 acts as a seal, preventing any flowof reactant away from the porous surface 20 while also preventing anyflow of coolant toward reactant channels 18 or porous surface 20. Theporous region 14 further defines a flow field medium for diffusing areactant (fuel or oxidant) to a porous surface 20 upon which is disposeda conventional catalyst/electrolyte arrangement 22.

The component is subsequently attached to the opposite electrode of asecond cell in a conventional series. Coolant channels 16 can be formedas partial channels on a surface of the component as grooves which alignwith similar grooves in an opposing fuel cell component to form completecoolant channels. Thus it is seen that the present invention providesthe combination of two components, the bipolar plate and the diffuser,into a single, simply fabricated component.

Another, more advanced embodiment combines two components as describedabove, as shown in FIG. 2. Back-to-back bipolar plate/diffusers arefabricated as one component, with coolant channels 32 formed as completechannels within the component, as well as reactant channels 18. Thehermetic region 34 defines coolant channels 32.

Since there are two porous regions 36, 38 in this embodiment, CVI toform the hermetic region 34 is accomplished by flowing the stream ofgaseous reactants through the coolant channels. Fuel channels 40 andoxidant channels 42 are at least partially defined by the two respectiveporous regions 36, 38 which further define flow field media fordiffusing a fuel and oxidant in opposite directions to respective poroussurfaces 44, 46 upon which are disposed respective catalyst/electrolytearrangements 48, 50. Thus it is seen that the present invention furtherprovides the combination into one component of two opposing combinationbipolar plate/diffuser components taught in the first embodiment; i.e.,two sets of bipolar plates and diffusers—four discrete components—havebeen combined into one component.

FIG. 2 further shows two such components 52, 54 in a stackedarrangement. Fuel from a fuel channel 40 of one component 54 and oxidantfrom an oxidant channel 42 of the next component 52 flow toward eachother and react at the catalyst/electrolyte arrangement 50 to produceelectricity. The simplicity of such an arrangement is evidenced in thatthe only two components necessary to complete a cycle in the stack arethe bipolar plate/diffusers 52, 54 and the catalyst/electrolytearrangements 48, 50.

The advantages of the invention include avoidance of deleterious fluidleaks and ohmic losses generally associated with conventional discretebipolar plate and diffuser arrangements which require multiple discretecomponents for each cycle in the stack thereof.

The bipolar plate/diffuser component is of benefit for low-cost, highvolume production of PEMFCs. These are useful in stationary powerproduction facilities, direct electric transportation vehicles,auxiliary power for transportation vehicles, and backup power systems.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications can be madetherein without departing from the scope of the inventions defined bythe appended claims.

What is claimed is:
 1. A method of making a monolithic combinationbipolar plate/diffuser fuel cell component comprising the steps of: a.mixing carbon and binder to form an electrically conducting homogeneousmaterial; b. screening said homogeneous material; c. molding saidhomogeneous material into a monolithic component having; a porous regionhaving a porous surface, at least one coolant channel, and at least onereactant channel; d. densifying said component; e.pyrolizing/carbonizing said component; f. infiltrating a portion of saidporous region with a carbon material to form a hermetic region of thebipolar plate/diffuser fuel cell component.
 2. A method of claim 1wherein said porous region and said hermetic region are generallyplanar.
 3. A method of claim 1 wherein said component further comprisesa solid catalyst/electrolyte arrangement disposed in operablecommunication with said porous surface.
 4. A method of claim 1 whereinsaid homogeneous material is an aqueous slurry comprising a mixture ofcarbon fibers having lengths typically in the range of about 0.1 mm toabout 100 mm and phenolic resin powder binder typically in the range ofabout 20% to about 50%.
 5. A method of claim 1 wherein said screeningstep removes solids thereby producing a wet preform which issubsequently dried at a temperature of less than 80° C. typicallyresulting in initial component porosity in the range of 70%-90%.
 6. Amethod of claim 1 wherein said molding step is slurry molding.
 7. Amethod of claim 1 wherein said densifying step is accomplished usingdrying and curing techniques.
 8. A method of claim 1 wherein saiddensifying step is accomplished using mechanical compression at lowpressure in shaped graphite molds at a temperature in the range of about120° C. to about 160° C.
 9. A method of claim 1 wherein saidpyrolizing/carbonizing step is accomplished by heating said component inan inert atmosphere to a temperature in the range of about 700° C. toabout 1300° C. resulting in a total porosity in the range of 40% to 60%and a pore size in the range of about 10 to about 100 microns.
 10. Amethod of making a combination bipolar plate/diffuser fuel cellcomponent comprising the steps of: a. mixing an electrically conductinghomogeneous material; b. screening said homogeneous material; c. moldingsaid homogeneous material into a monolithic component having; a firstporous region having a porous surface, a second porous region having aporous surface, a hermetic region, at least one coolant channel, and atleast one reactant channel d. densifying said component; e.pyrolizing/carbonizing said component; f. infiltrating said hermeticregion with a densified carbon material.
 11. A method of claim 10wherein said first porous region, said second porous region and saidhermetic region are generally planar.
 12. A method of claim 10 whereinsaid component further comprises a solid catalyst/electrolytearrangement disposed in operable communication with said first andsecond porous surfaces.
 13. A method of claim 10 wherein saidhomogeneous material is an aqueous slurry comprising a mixture of carbonfibers having lengths typically in the range of about 0.1 mm to about100 mm and phenolic resin powder binder typically in the range of about20% to about 50%.
 14. A method of claim 10 wherein said screening stepremoves solids thereby producing a wet preform which is subsequentlydried at a temperature of less than 80° C. typically resulting ininitial component porosity in the range of 70%-90%.
 15. A method ofclaim 10 wherein said molding step is slurry molding.
 16. A method ofclaim 10 wherein said densifying step is accomplished using drying andcuring techniques.
 17. A method of claim 10 wherein said densifying stepis accomplished using mechanical compression at low pressure in shapedgraphite molds at a temperature in the range of about 120° C. to about160° C., preferably about 130° C.
 18. A method of claim 10 wherein saidpyrolizing/carbonizing step is accomplished by heating said component inan inert atmosphere to a temperature in the range of about 700° C. toabout 1300° C. resulting in a total porosity in the range of 40% to 60%and a pore size in the range of about 10 to about 100 microns.